1. Field of the Invention
The present invention generally relates to a disk drive of a perpendicular magnetic recording system, and particularly to a servo data coded recording system.
2. Description of the Related Art
In recent years, in the field of disk drives represented by a hard disk drive or the like, a perpendicular magnetic recording method is noted. This method enables an increase in the density of recording as compared with the conventional longitudinal magnetic recording method.
In the longitudinal magnetic recording method, as shown in FIG. 16A, in the case where data (bit “0” or “1”) is recorded on the data track 160 on the disk, a magnetized area (shown by an arrow 162) corresponding to the data is formed in a longitudinal direction (corresponding to the rotation direction 161) of the disk. FIG. 16B is a view showing a read signal waveform read with a head (magnetic head).
In the disk drive in which the perpendicular magnetic recording method is adopted, the read signal read from the disk includes low frequency components (DC components). Consequently, the read signal is liable to be affected by the cut-off low frequency characteristic of the read/write channel, so that a waveform distortion tends to be easily generated in the read signal waveform. Consequently, at the time of the processing of the PRML method signal processing in the read/write channel, error data is detected so that there is a possibility that the read error rate is heightened. Conventionally, there is proposed a technique for preventing a decoding error on the receiving side at the time of transmitting coded data including direct current error (see, for example, Japanese Unexamined Patent Publication No. 5-4866). However, it is hard to apply the prior art to the perpendicular magnetic recording method disk drive as it is.
Furthermore, in the disk drive, the read signal includes low-frequency servo data together with relatively high frequency user data. Consequently, the read signal corresponding to the servo data has a signal waveform which is liable to be affected by the influence of the cut-off low frequency characteristic of the read/write channel. When the read error rate of the servo data is heightened, the possibility becomes higher that the decoded data of the servo address includes error data. Since a servo address is used in the head position control, the generation of the servo address 2 will lead to a reduction in the precision of the head positioning control (that is, performance of the servo system). As a consequence, in the perpendicular magnetic recording system, the reduction in the performance of the servo system becomes one problem which hinders the practical use of the servo system.